Carbon nanotubes (CNT) are cylindrical carbon molecules with a diameter of a few nanometers. Due to their tubular structure and unique dimensions, carbon nanotubes exhibit remarkable electrical, thermal, chemical and mechanical properties. In the last decade, the CNTs have become a very important material for different domains like electronics, optoelectronics, automation and control fields, for current or potential applications such as: electrochemical capacitors, sensors (gas sensors and biosensors), field-emission displays, solar cells, transistors, Schottky diodes, photovoltaic cells and photodiodes, composite fiber in polymers and ceramics. The CNT can be classified into two types: single-walled carbon nanotubes and multi-walled carbon nanotubes.
Manufacturing of CNT based devices is still in the early stages due to present technological challenges like: insolubility of parent carbon nanotubes in most common organic solvents and low processing capabilities. Recently, in order to improve CNT solubility in water and organic solvents, and also the chemical compatibility with polymers and ceramics, CNT functionalization has been proposed as a possible technological thrust. Noncovalent functionalization implies π-π stacking interactions between the surface of carbon nanotubes and aromatic molecules such as benzene or styrene. As a major advantage, non-covalent functionalization preserves mechanical and electrical properties. However, the forces between carbon nanotubes and wrapping molecules may be weak. In the case of covalent functionalization, the attachment of molecule at the surface of carbon nanotubes is strong (covalent bond), but this type of functionalization introduces defects in the structure of CNTs and can affect the electrical and mechanical properties of CNTs. For example, if the carbon nanotubes are sonicated in the presence of detergent molecules they disperse to create a suspension with limited stability. Several procedures have been developed in order to modify the structure of carbon nanotubes and to link at its surface some reactive groups such as: carboxylic groups, phenolic groups, amino groups, etc.
The derivatization through covalent functionalization of carbon nanotubes (single walled and multi-walled carbon nanotubes) with amino groups (or with amino and sulfonic groups) opens perspectives for the synthesis of novel carbon nanotubes which can be the precursors for the design of new molecular architectures. Amino carbon nanotubes can be synthesized by sonicating carbon nanotubes in HNO3 in order to create carboxyl groups at the surface, treating the resulting carboxycarbon nanotubes with thionyl chloride and synthesizing the corresponding amino carbon nanotubes through the intermediate of ammonia. Finally, Hoffman degradation in presence of natrium hypo bromide yields desirable amino carbon nanotubes.
An alternative procedure of CNT functionalization consists in the reaction between the acid chloride and carbon nanotubes with sodium azide followed by Curtius rearrangement. Also, synthesis of single-walled carbon nanotubes with amino methyl group (CH2NH2) has been reported. Recent publications relate synthesis of amino carbon nanotubes (single-walled and multi-walled) by means of nucleophilic substitution from fluorinated carbon nanotubes (synthesized from carbon nanotubes and fluorine at temperature>1500C) and alkyl amine. Additionally, it is possible to connect two fluorinated carbon nanotubes in a nucleophilic substitution reaction with a, w aliphatic diamine such as hexamethyleneamine, cadaverine or putresceine.
Many efforts have been made in order to improve the mechanical, electrical and chemical properties of polymers. Thus, one direction was to incorporate (carbon nanotubes (single-walled or multi-walled)) in the bulk of polymers. In the preparation of matrix composite CNTs/polymer, CNTs (modified or unmodified) were combined with conducting organic polymers (such as polyanilines (PANI), poly (3-,4-ethylenedioxy thiophenes), polypyrroles, polythiophenes, poly (p-phenylene vinylene), or with insulators polymers such as polycarbonates, polyethylene terephthalates, polystyrenes, polyphenylenesulphides, polysulfones, nylons, or copolymers such as poly (butylene adipate)-co-(amino caproate).
Incorporation of carbon nanotubes in polymers represents a remarkable way to improve electrical properties (in the case of conducting polymers through the π-π stacking interactions), or mechanical properties. Moreover if the carbon nanotubes are added in the host matrix of polymer it is possible to tailor the chemical properties or physical properties for future applications (sensitivity for different type of gas molecules, compression modulus, and capacity of energy storage).
Like fullerenes, carbon nanotubes have high electron affinity and thus can act as an agents trapping radicals. Due to this property CNTs have a similar behavior with that of an antioxidant. Boron-doped carbon nanotubes can be used with good results for such a role too. Thus incorporation of CNTs in polymeric materials decreases the level of free radicals and increases the lifetime of polymers.
Carbon dioxide is a molecule with low reactivity, which and this is why it is difficult to sense. The sensing of carbon dioxide has a paramount importance in a broad variety of applications in hospitals (for capnography), in the management of building air, in chemical industry and in agriculture. For these applications the current technology-infrared spectroscopy is limited by its power consumption and size. Detection of carbon dioxide with SAW/BAW sensors represents an alternative which offers, for example, great sensitivity. In order to satisfy these demands active efforts have been made in the recent years to develop new CO2-sensitive coatings for SAW/BAW sensors.
Phthalocyanine (PC) can be useful for carbon dioxide detection. Versamid 900, Polyethyleneimine (PEI), and BMBT (N,N bis-(p-methoxybenzylidene)-α-{acute over (α)}′-bi-p-toluidine represent three polymers which were tested for the detection of carbon dioxide with SAW sensors. Among these polymers, the present inventors have found that PEI seems to show the best sensitivity (a frequency shift of about 1 kHz was measured for a concentration of 240 ppm of carbon dioxide in nitrogen). Primary, secondary and tertiary amino groups that exist in PEI are responsible enhanced sensing because amino groups react at room temperature with carbon dioxide and yield carbamates. The response time is of a few seconds and the chemical reaction is reversible.